1. Field of the Invention
The present invention is broadly concerned with extruded products containing or consisting essentially of a starch-bearing material, such as pure starches, legumes or grains wherein the products are non-sticky and exhibit low cold water viscosities and high cook values. More particularly, the invention is concerned with such products and methods of producing the products wherein a starting material, such as a starch-bearing grain, is initially treated by heat and moisture to partially cook the material, followed by low-shear extrusion. The total specific thermal energy (STE) and specific mechanical energy (SME) inputs from the method give a total STE/SME ratio of at least about 4. These products have functionalities equivalent to those conventionally produced by batch processes
2. Description of the Prior Art
Extrusion processing of mixtures, such as human foods or animal feeds, is well known and widely practiced. In general, extrusion processing of foods involves passing a comestible formulation into and through an elongated extruder barrel equipped with one or more elongated, helically flighted, axially rotatable screws and an endmost restricted orifice die. Steam and/or water may be injected into the barrel during such processing, and the barrel may be indirectly heated or cooled using heat exchange media. Often, the formulations are preconditioned prior to extrusion by moisturizing and heating the formulations in a specialized preconditioner device. In general, extrusion processes provide a desirable degree of cook in the final product by a combination of specific mechanical energy and specific thermal energy inputs. Typical high shear extruders have an STE/SME ratio of below 1, meaning that most of the energy is in the form of SME.
Attempts have been made to produce extruded products having starch-bearing grain(s) as the predominant or entire fraction of the starting ingredients. For example, corn or wheat flours have been extruded in an effort to produce end products suitable for use as ingredients in foods such as breads or the like. These efforts have not been successful. Instead, extruded products of this character tend to be very sticky and difficult to work, and give a “slimy” mouth feel when consumed. Accordingly, such products are conventionally produced using expensive batch processes and not by extrusion, even though the latter would be much less expensive.
Prior researchers have determined that the extent of shear in typical extrusion cooking gives starch-bearing grains very high cold water viscosities, which are one cause of the stickiness and mouth feel problems. See, e.g., Majzoobi et al., Effect of Extrusion Cooking on the Molecular and Physical Properties of Cereal Starches, 4th International Symposium on Food Rheology and Structure (2006). In addition, U.S. Pat. No. 6,422,135 describes a process for producing free-gel starch products in a low shear extrusion system. However, the methods described in the '135 patent have total STE/SME ratios on the order of 2.5-2.7, and many of the products have high cold water solubilities. Processing according to the '135 patent is also relatively expensive.